Lizard or bird? Defining Archaeopteryx computer tomography

LONDON – Paleontologists look for similarities between long-extinct species and present-day animals to gain insight into the appearance and behavior of prehistoric beasts. Soft tissue rarely is preserved in the fossil record, however, so most of their observations rely on interpretations of the bones or on the empty spaces that once housed other structures.

A digital endocast of a common barn owl shows how scientists were able to measure the dimensions of the cochlear duct. Images courtesy of and copyright by the Natural History Museum.
For birds, amphibians and reptiles, the organ used in hearing – the basilar papilla – is a piece of soft tissue. It is housed within the bony part of the inner ear (the cochlear duct), however, so scientists at the Natural History Museum, the Technical University of Munich in Germany and Ohio University College of Osteopathic Medicine in Athens began looking for a possible correlation between that bone and the hearing range of the species to which it belonged. They used CT imaging to noninvasively create a three-dimensional model of the duct in bird and reptile species with known hearing ranges, comparing the results to determine whether there was a correlation.

This fossil gives a good general idea as to the appearance of the Archaeopteryx. According to Dr. Paul M. Barrett, a paleontologist at the museum, the results were surprising. Although previous investigations of auditory physiology had suggested that the bony parts of the ear were largely irrelevant to hearing, the researchers found that there was a strong correlation between the cochlear duct and the hearing range, and that the findings could be applied to a large range of animal species. The research indicates that animals that are more highly vocal and social, with a wider range of hearing, have statistically longer ducts.

Previously, scientists estimated how prehistoric animals heard by examining damaged fossil skulls and comparing the sizes of the brain regions to those in modern animals. This group, however, applied its research into the cochlear duct to fossils as well.

An artist’s rendering of the prehistoric Archaeopteryx.
The researchers took 3-D CT images of the skull of an Archaeopteryx, a magpie-size creature that lived approximately 145 million years ago. Although the animal has a fairly reptilian appearance, with a long tail and teeth that resemble those of modern lizards, the investigation into its ear structure revealed that its hearing range was about 2000 Hz – roughly equivalent to that of an emu.

The discovery enabled the investigators to make inferences about the vocal complexity and possible social structure of the prehistoric beast.